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Colossal Squid

I always wondered why four was such a "magic" number... Vertebrates either have two (birds, although I guess wings count as appendages...) or four limbs (the rest of them...), unless you count snakes (which ancestrally had four...). Other invertebrates usually either have many (centipedes, millipedes) or 6 (insects). Why not 8? Why is it that cephs and arachnids are the only ones that have 8? Hmm... must go do some reading...

Yeah, but they cheat; developmentally, they're not true radiates, they're bilats who grab their tails with their heads and fuse.

I think the basic animal body plan controlled by HOX genes and the like strongly favors bilateral symmetry and has for a long time; and even recently, it looks like true radiates like cniderians are even based on the same body patterns. So really it's only really primitive things like sponges that aren't based on this. So pretty much anything ambulatory is likely to have things in symmetric pairs... I would be very curious if anyone's studied the genetics of echinoderm development enough to understand what signals the major morphological differences in "inside side" vs "outside side" when they take their adult forms. Other animals have some degree of violation of the bilaterality, though, so it may just be a more dramatic manifestation of the same mechanism.

Anyway, from studying a bit about animal and robot locomotion, I can point out that for things on feet, if you have 4 legs, you can always keep your center of gravity over a triangle of 3 legs while you move the 4th, so it's a lot easier to be stable and not fall over as you walk. Us bipeds have to consistently be in danger of falling over; in fact, walking is really more like continually falling forward and catching yourself, which is not true of quadropeds. Sometimes there is a slight gain in going to 6 legs in this regard, since you don't have to shift your center of mass so much to take the weight off a leg to step; there's a "hexapod" robot someone made to take advantage of this. I'm not sure why there's an advantage in some animals to having more legs, though... roach walking has been studied a fair bit, and they seem to just get more speed out of being able to have more legs in action at a given time. I'm not aware of any theories on why 8 or 10 limbs in cephs, or more for nautilus, or why it's an advantage for spiders or milipedes to have more than 4...

I'm hoping Fujisawa Sake-san will chime in here, it seems like this is right up his alley... however, he's been quiet lately, too quiet...

Colossal Squid

Great, now I am going to have to dig out my development books... I know that echinoderm larva have bilateral symmetry, but doesn't the adult stage develop inside of the larvae? Off to do some reading...

TONMO Supporter

Great, now I am going to have to dig out my development books... I know that echinoderm larva have bilateral symmetry, but doesn't the adult stage develop inside of the larvae? Off to do some reading...

Echinoderms evolved from bilaterally symmetric creatures. Later forms were lopsided. Echinoderms' larvae are ciliated free-swimming organisms that organize in a bilaterally symmetric fashion that makes them look like embryonic chordates. Later, the left side of the body grows at the expense of the right side, which is eventually absorbed. The left side then grows in a pentaradially symmetric fashion, in which the body is arranged in five parts around a central axis.

Colossal Squid

I can't find my development books right now (must not have unpacked them yet... only moved here 11 months ago...).

My invert text says that 50% of echinoderms have direct development. In the others, the left side of the larvae becomes the oral surface and the right side becomes the aboral. In sea stars, the larvae undergoes metamorphsis. The mouth, esophagus, part of the intestine and anus degenerate and then are formed again in a radial symmetrical fashion in the baby sea star.

At first I was going to comment that the octopus only took up the walking around on the bottom lifestyle comparatively recently, but then I realized that most models of how the first shelled cephs developed was that some limpet-like critter was on the bottom and gained the ability to use its shell to become buoyant. One might imagine that as it became lighter, it could move a lot faster by walking on its arms than by using a gastropodish foot, even if it wasn't a free swimmer yet... in fact, maybe it was the walking faster and being a more active predator that came first, and the floating initially was an adaptation to be more effective at that...

And yes, ing that link and the nature paper are very inducing... definitely puts me back in my place as an official "ignorant computer scientist" rather than a real bio- or paleo- expert!

That's true, but when octos are using their arms for locomotion, they're mostly in contact with the bottom or with whatever rock/cave/reef they're involved with, so they're not really any more 3-d than squirrels or monkeys. Of course, octos can free-swim, too, but they don't use their arms most of the time when they're doing, that-- and squids, who spend their whole lives in the 3-d world, don't use their arms and tentacles for locomotion at all... and they have ten limbs!

Also, most snails can only crawl forward, they can turn and move another direction, but it is still head forward.

Colossal Squid

Originally posted by Monty
That's true, but when octos are using their arms for locomotion, they're mostly in contact with the bottom or with whatever rock/cave/reef they're involved with, so they're not really any more 3-d than squirrels or monkeys. Of course, octos can free-swim, too, but they don't use their arms most of the time when they're doing, that-- and squids, who spend their whole lives in the 3-d world, don't use their arms and tentacles for locomotion at all... and they have ten limbs!

Titanites

Us bipeds have to consistently be in danger of falling over; in fact, walking is really more like continually falling forward and catching yourself, which is not true of quadropeds. Sometimes there is a slight gain in going to 6 legs in this regard, since you don't have to shift your center of mass so much to take the weight off a leg to step; there's a "hexapod" robot someone made to take advantage of this.

I'm pretty sure this is at the heart of my fear of spiders -- they seem almost overly-equipped and dominant in that they are in full control; moreso than me. And perhaps that doesn't bother me so much with octopuses since they don't have to contend with gravity in the same way.

Colossal Squid

I'm pretty sure this is at the heart of my fear of spiders -- they seem almost overly-equipped and dominant in that they are in full control; moreso than me. And perhaps that doesn't bother me so much with octopuses since they don't have to contend with gravity in the same way.

Larger Pacific Striped Octopus

Hmm.. This is very interesting. See, in Cosmos, Carl Sagan made an interesting note about how the tetrapod body plan was so prevalent among vertebrates, and how this was probably due to the overall tetra-well, tetra-'fin' body plan of our lobe-finned teleost ancestors. The five-rayed "hand" comes from reduction of the main bony rays of the fins, and so on...

Here's the issue that some find troubling: its probably just by chance that there are "magic numbers". I know we try, unconsciously for some, religiously for others, to find patterns and directions in evolution. We look for an arrow, as if everything is evolving toward some grand design. If those aforementioned rays had been reduced to four, then maybe many things would have been different. Imagine if the events leading to the end-Permian event had been different. It would seem evolution happens a lot by chance.

Monty had an interesting idea with the Hox Genes. See, linkage is an important factor in evolution as well. And entire phenotypes (expressed genes - eye color, hair color, etc.) can change with one gene. This happens because gene effects can cascade - one gene can affect many. Which is why a small mutation can have mostrous, even lethal changes. It also explains why something like punctuated equilibrium, or rapid evolutionary change, is prevalent in the fossil record.

I know this sounds like a cop-out, but I have to think that the evolution of limbs and symmetry is a mixture of chance happenings - environmental & genetic - along the evolutionary line. I mean, yes, octopus-neurology has evolved a remarkable control system for eight legs, but was this due to having eight legs, or was the evolution of eight legs due to the neurology being better adapted for eight legs, or was it both happening at the same time? There's the $10,000 question.